added method to generate an time expiring url

(cherry picked from commit 61dbcb3e36c0bc9742d045dd1e54f846043a1569)

lib/paperclip/storage.rb

@@ -160,6 +160,10 @@ module Paperclip
         end
       end
       
+      def expiring_url(time = 3600)
+        AWS::S3::S3Object.url_for(path, bucket_name, :expires_in => time )
+      end
+
       def bucket_name
         @bucket
       end

test/storage_test.rb

@@ -97,6 +97,33 @@ class StorageTest < Test::Unit::TestCase
     end
   end
   
+  context "Generating a url with an expiration" do
+    setup do
+      AWS::S3::Base.stubs(:establish_connection!)
+      rebuild_model :storage => :s3,
+                    :s3_credentials => {
+                      :production   => { :bucket => "prod_bucket" },
+                      :development  => { :bucket => "dev_bucket" }
+                    },
+                    :s3_host_alias => "something.something.com",
+                    :path => ":attachment/:basename.:extension",
+                    :url => ":s3_alias_url"
+                    
+      rails_env("production")
+      
+      @dummy = Dummy.new
+      @dummy.avatar = StringIO.new(".")
+      
+      AWS::S3::S3Object.expects(:url_for).with("avatars/stringio.txt", "prod_bucket", { :expires_in => 3600 })
+      
+      @dummy.avatar.expiring_url
+    end
+    
+    should "should succeed" do
+      assert true
+    end
+  end
+
   context "Parsing S3 credentials with a bucket in them" do
     setup do
       AWS::S3::Base.stubs(:establish_connection!)